The electrical conductivity of the oceanic upper mantle

Previous inversions of sea-floor magnetotelluric (MT) sounding data have predicted upper mantle electrical conductivities which are more than an order of magnitude higher than laboratory measurements of the conductivity of olivine would suggest, and controlled source-field electromagnetic (CSEM) soundings require a lithos-pheric mantle conductivity of 3 x lop5 Sm-’, which is so low that the electromag-netic (EM) coast effect would produce more anisotropy in MT soundings than is observed. We address these issues by constructing an olivine mantle model for conductivity and examining the inversion of MT data from three sea-floor sites, and show that the incompatibilities can be largely resolved if the effects of oceans and coastlines are considered. Our mantle model is based on recent measurements of olivine conductivity, the conductivity of tholeiite melt, a thermal model for the upper mantle based on lithospheric cooling and the temperature of a+ a + /3 olivine transition, the pyrolite model of mantle petrology, and conductivities derived from CSEM sounding. We propose Archie’s Law with exponent 2 and intercon-nected tubes as realistic lower and upper bounds for the effect of partial melt o